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Derivatives And Risk Management (Narrative) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Derivative [Line Items]    
Derivative, Fair Value, Amount Offset Against Collateral, Net $ 22,371 $ 62,408
Commodity Contracts [Member]    
Derivative [Line Items]    
Cash deposited as collateral 29,757 39,458
Letters of credit outstanding 21,700 23,000
Derivative, Fair Value, Amount Offset Against Collateral, Net 21,781 27,438
Liability position at aggregate fair value 1,529 1,336
Additional Collateral, Aggregate Fair Value 1,529 1,336
Interest Rate Swap [Member]    
Derivative [Line Items]    
Cash deposited as collateral 590 34,970
Letters of credit outstanding 0 5,000
Derivative, Fair Value, Amount Offset Against Collateral, Net 590 34,970
Liability position at aggregate fair value 5,491 73,514
Additional Collateral, Aggregate Fair Value $ 2,400 $ 18,770